Wire grid polarizers may be used for polarizing light, by allowing one polarization of light to pass through the polarizer, and reflecting or absorbing an opposite polarization of light. For simplicity, the polarization that primarily passes through the polarizer will be referred to as p-polarized light and the polarization that primarily is reflected or absorbed will be referred to as s-polarized light. Goals of wire grid polarizer design include increasing transmission of p-polarized light, decreasing transmission of s-polarized light, and increasing reflection or absorption of s-polarized light. Different applications have different requirements.
The goals of increasing transmission of p-polarized light and decreasing transmission of s-polarized light are common to most or all applications. There can be a trade-off between these two. In other words, certain designs that may increase transmission of p-polarized light may also undesirably increase transmission of s-polarized light. Other designs that decrease transmission of s-polarized light may also undesirably decrease transmission of p-polarized light.
For some applications, it is desirable to reflect as much s-polarized light as possible. For example, if s-polarized light is primarily reflected, then the optical system can effectively utilize both the transmitted p-polarized light and the reflected s-polarized light. It can be important in such designs to increase reflection of s-polarized light without reducing transmission of p-polarized light. Sometimes there is a trade-off in a particular design between increasing transmission of p-polarized light and increasing reflection of s-polarized light.
For other applications, absorption of s-polarized light may be preferred. Absorption of s-polarized light may be preferred if the reflection of light can disrupt the image or other intended use. For example, in a transmissive panel image projection system, reflected light may go back into the LCD imager causing image degradation, or stray light can reach the screen, degrading contrast. An ideal selectively absorptive wire grid polarizer will transmit all p-polarized light and selectively absorb all s-polarized light. In reality, some s-polarized light is transmitted and some reflected and some p-polarized light is absorbed and some reflected. Sometimes there is a trade-off in a particular design between increasing transmission of p-polarized light and increasing absorption of s-polarized light.
The effectiveness of a wire grid polarizer can thus be quantified by (1) high transmission of p-polarized light; (2) high absorption or reflection of s-polarized light, depending on the design; and (3) high contrast. Contrast is equal to percent of p-polarized light transmitted (Tp) divided by percent of s-polarized light transmitted (Ts): Contrast=Tp/Ts.
It can be important in wire grid polarizers for infrared, visible, and ultraviolet light to have small wires with small pitch, such as nanometer or micrometer size and pitch, for effective polarization. Typically, a pitch of less than half of the wavelength of light to be polarized is needed for effective polarization. Smaller pitches may improve the contrast. Thus, small pitch can be an important feature of wire grid polarizers. Manufacture of wire grid polarizers with sufficiently small pitch is challenging, and is a goal of research in this field.
Small wires can be damaged by handling and by environmental conditions. Protection of the wires can be important in wire grid polarizers. Durability of wire grid polarizers is thus another important feature.
For example, see U.S. Pat. Nos. 5,991,075, 6,288,840, 6,665,119, 7,630,133, 7,692,860, 7,800,823, 7,961,393, and 8,426,121; U.S. Patent Publication Numbers US 2008/0055723, US 2009/0041971, and US 2009/0053655; U.S. patent application Ser. No. 13/326,566, filed on Dec. 15, 2011; “Application of 100 Å linewidth structures fabricated by shadowing techniques” by D. C. Flanders in J. Vac. Sci. Technol., 19(4), November/December 1981; and “Submicron periodicity gratings as artificial anisotropic dielectrics” by Dale C. Flanders in Appl. Phys. Lett. 42 (6), 15 Mar. 1983, pp. 492-494.